Movies and television have presented a romantic vision of archaeology as adventure in far-away and exotic locations. A more realistic picture might show researchers digging in smelly mud for hours under the hot sun while battling relentless mosquitoes. This type of archaeological research produces hundreds of small plastic bags containing pottery shards, animal bones, bits of worked stone, and other fragments. These findings must be classified,  which requires more hours of tedious work in a stuffy tent. At its best, archaeology involves a studious examination of the past with the goal of learnting important information about the culture and customs of ancient (or not so ancient) peoples. Much archaeology in the early twenty-first century investigates the recent past, a sub-branch called “historical archaeology.”

What Is Archaeology?

Archaeology is the study of the material remains of past human cultures. It is distinguished from other forms of inquiry by its method of study, excavation. (Most archaeologists call this “digging.”) Excavation is not simply digging until something interesting is found. That sort of unscientific digging destroys the archaeological information. Archaeological excavation requires the removal of material layer by layer to expose artifacts in place. The removed material is carefully sifted to find small artifacts, tiny animal bones, and other remains. Archaeologists even examine the soil in various layers for microscopic material, such as pollen. Excavations, in combination with surveys, may yield maps of a ruin or collections of artifacts.

Time is important to archaeologists. There is rarely enough time to complete the work, but of even greater interest is the time that has passed since the artifact was created. An important part of archaeology is the examination of how cultures change over time. It is therefore essential that the archaeologist is able to establish the age of the artifacts or other material remains and arrange them in a chronological sequence. The archaeologist must be able to distinguish between objects that were made at the same time and objects that were made at different times. When objects that were made at different times are excavated, the archaeologist must be able to arrange them in a sequence from the oldest to the most recent.

Relative Dating and Absolute Dating

Before scientific dating techniques such as dendrochronology and radiocarbon dating were introduced to archaeology, the discipline was dominated by extensive discussions of the chronological sequence of events. Most of those questions have now been settled and archaeologists have moved on to other issues. Scientific dating techniques have had a huge impact on archaeology.

Archaeologists use many different techniques to determine the age of an object. Usually, several different techniques are applied to the same object. Relative dating arranges artifacts in a chronological sequence from oldest to most recent without reference to the actual date. For example, by studying the decorations used on pottery, the types of materials used in the pottery, and the types and shapes of pots, it is often possible to arrange them into a sequence without knowing the actual date. In absolute dating, the age of an object is determined by some chemical or physical process without reference to a chronology.

Relative Dating Methods.

The most common and widely used relative dating technique is stratigraphy. The principle of superposition (borrowed from geology) states that higher layers must be deposited on top of lower layers. Thus, higher layers are more recent than lower layers. This only applies to undisturbed deposits. Rodent burrows, root action, and human activity can mix layers in a process known as bioturbation. However, the archaeologist can detect bioturbation and allow for its effects.

Discrete layers of occupation can often be determined. For example, Hisarlik, which is a hill in Turkey, is thought by some archaeologists to be the site of the ancient city of Troy. However, Hisarlik was occupied by many different cultures at various times both before and after the time of Troy,  and each culture built on top of the ruins of the previous culture, often after violent conquest. Consequently, the layers in this famous archaeological site represent many different cultures. An early excavator of Hisarlik, Heinrich Schleimann, inadvertently dug through the Troy layer into an earlier occupation and mistakenly assigned the gold artifacts he found there to Troy. Other sites have been continuously occupied by the same culture for a long time and the different layers represent gradual changes. In both cases, stratigraphy will apply.

A chronology based on stratigraphy often can be correlated to layers in other nearby sites. For example, a particular type or pattern of pottery may occur in only one layer in an excavation. If the same pottery type is found in another excavation nearby, it is safe to assume that the layers are the same age. Archaeologists rarely make these determinations on the basis of a single example. Usually, a set of related artifacts is used to determine the age of a layer.

Seriation simply means ordering. This technique was developed by the inventor of modern archaeology, Sir William Matthew Flinders Petrie. Seriation is based on the assumption that cultural characteristics change over time. For example, consider how automobiles have changed in the last 50 years (a relatively short time in archaeology). Automobile manufacturers frequently introduce new styles about every year, so archaeologists thousands of years from now will have no difficulty identifying the precise date of a layer if the layer contains automobile parts.

Cultural characteristics tend to show a particular pattern over time. The characteristic is introduced into the culture (for example, using a certain type of projectile point for hunting or wearing low-riding jeans), becomes progressively more popular, then gradually wanes in popularity. The method of seriation uses this distinctive pattern to arrange archaeological materials into a sequence. However, seriation only works when variations in a cultural characteristic are due to rapid and significant change over time. It also works best when a characteristic is widely shared among many different members of a group. Even then, it can only be applied to a small geographic area, because there is also geographic variation in cultural characteristics.

For example, 50 years ago American automobiles changed every year while the Volkswagen Beetle hardly changed at all from year to year.

Cross dating is also based on stratigraphy. It uses the principle that different archaeological sites will show a similar collection of artifacts in layers of the same age. Sir Flinders Petrie used this method to establish the time sequence of artifacts in Egyptian cemeteries by identifying which burials contained Greek pottery vessels. These same Greek pottery styles could be associated with monuments in Greece whose construction dates were fairly well known. Since absolute dating techniques have become common, the use of cross dating has decreased significantly.

Pollen grains also appear in archaeological layers. They are abundant and they survive very well in archaeological contexts. As climates change over time, the plants that grow in a region change as well. People who examine pollen grains (the study of which is known as pollen analysis) can usually determine the genus, and often the exact species producing a certain pollen type. Archaeologists can then use this information to determine the relative ages of some sites and layers within sites. However, climates do not change rapidly, so this type of analysis is best for archaeological sites dating back to the last ice age.

Absolute Dating Methods.

Absolute dating methods produce an actual date, usually accurate to within a few years. This date is established independent of stratigraphy and chronology. If a date for a certain layer in an excavation can be established using an absolute dating method, other artifacts in the same layer can safely be assigned the same age.

Dendrochronology, also known as tree-ring dating, is the earliest form of absolute dating. This method was first developed by the American astronomer Andrew Ellicott Douglas at the University of Arizona in the early 1900s. Douglas was trying to develop a correlation between climate variations and  sunspot activity, but archaeologists quickly recognized its usefulness as a dating tool. The technique was first applied in the American Southwest and later extended to other parts of the world.

Tree-ring dating is relatively simple. Trees add a new layer of cambium (the layer right under the bark) every year. The thickness of the layer depends on local weather and climate. In years with plenty of rain, the layer will be thick and healthy. Over the lifetime of the tree, these rings accumulate, and the rings form a record of regional variation in climate that may extend back hundreds of years. Since all of the trees in a region experience the same climate variations, they will have similar growth patterns and similar tree ring patterns.

One tree usually does not cover a period sufficiently long to be archaeologically useful. However, patterns of tree ring growth have been built up by “overlapping” ring sequences from different trees so that the tree ring record extends back several thousand years in many parts of the world. The process starts with examination of the growth ring patterns of samples from living trees. Then older trees are added to the sequence by overlapping the inner rings of a younger sample with the outer rings of an older sample.

Older trees are recovered from old buildings, archaeological sites, peat bogs, and swamps. Eventually, a regional master chronology is constructed.

When dendrochronology can be used, it provides the most accurate dates of any technique. In the American Southwest, the accuracy and precision of dendrochronology has enabled the development of one of the most accurate prehistoric cultural chronologies anywhere in the world. Often events can be dated to within a decade. This precision has allowed archaeologists working in the American Southwest to reconstruct patterns of village growth and subsequent abandonment with a fineness of detail unmatched in most of the world.

Radiometric dating methods are more recent than dendrochronology.

However, dendrochronology provides an important calibration technique for radiocarbon dating techniques. All radiometric-dating techniques are based on the well-established principle from physics that large samples of radioactive isotopes decay at precisely known rates. The rate of decay of a radioactive isotope is usually given by its half-life. The decay of any individual nucleus is completely random. The half-life is a measure of the probability that a given atom will decay in a certain time. The shorter the half-life, the more likely the atom will decay. This probability does not increase with time. If an atom has not decayed, the probability that it will decay in the future remains exactly the same. This means that no matter how many atoms are in a sample, approximately one-half will decay in one half-life. The remaining atoms have exactly the same decay probability, so in another halflife, one half of the remaining atoms will decay. The amount of time required for one-half of a radioactive sample to decay can be precisely determined.

The particular radioisotope used to determine the age of an object depends on the type of object and its age.

Radiocarbon is the most common and best known of radiometric dating techniques, but it is also possibly the most misunderstood. It was developed at the University of Chicago in 1949 by a group of American scientists led by Willard F. Libby. Radiocarbon dating has had an enormous impact on archaeology. In the last 50 years, radiocarbon dating has provided the basis for a worldwide cultural chronology. Recognizing the importance of this technique, the Nobel Prize committee awarded the Prize in Chemistry to Libby in 1960.

The physics behind radiocarbon dating is straightforward. Earth’s  atmosphere is constantly bombarded with cosmic rays from outer space. Cosmic-ray neutrons collide with atoms of nitrogen in the upper atmosphere, converting them to atoms of radioactive carbon-14. The carbon-14 atom quickly combines with an oxygen molecule to form carbon dioxide. This radioactive carbon dioxide spreads throughout Earth’s atmosphere, where it is taken up by plants along with normal carbon-12. As long as the plant is alive,the relative amount (ratio) of carbon-14 to carbon-12 remains constant at about one carbon-14 atom for every one trillion carbon-12 atoms. Some animals eat plants and other animals eat the plant-eaters. As long as they are alive, all living organisms have the same ratio of carbon-14 to carbon-12 as in the atmosphere because the radioactive carbon is continually replenished,  either through photosynthesis or through the food animals eat.

However, when the plant or animal dies, the intake of carbon-14 stops and the ratio of carbon-14 to carbon-12 immediately starts to decrease. The half-life of carbon-14 is 5,730 years. After 5,730 years, about one-half of the carbon-14 atoms will have decayed. After another 5,730 years, one-half of the remaining atoms will have decayed. So after 11,460 years, only onefourth will remain. After 17,190 years, one-eighth of the original carbon14 will remain. After 22,920 years, one-sixteenth will remain.

Radiocarbon dating has become the standard technique for determining the age of organic remains (those remains that contain carbon). There are many factors that must be taken into account when determining the age of an object. The best objects are bits of charcoal that have been preserved in completely dry environments. The worst candidates are bits of wood that have been saturated with sea water, since sea water contains dissolved atmospheric carbon dioxide that may throw off the results. Radiocarbon dating can be used for small bits of clothing or other fabric, bits of bone, baskets,

or anything that contains organic material.

There are well over 100 labs worldwide that do radiocarbon dating. In the early twenty-first century, the dating of objects up to about 10 half-lives, or up to about 50,000 years old, is possible. However, objects less than 300 years old cannot be reliably dated because of the widespread burning of fossil fuels,  which began in the nineteenth century, and the production of carbon-14 from atmospheric testing of nuclear weapons in the 1950s and 1960s. Another problem with radiocarbon dating is that the production of carbon-14 in the atmosphere has not been constant, due to variation in solar activity. For example,  in the 1700s, solar activity dropped (a phenomenon called the “Maunder Minimum”), so carbon-14 production also decreased during this period. To achieve the highest level of accuracy, carbon-14 dates must be calibrated by comparison to dates obtained from dendrochronology.

Calibration of Radiocarbon Dates.

 Samples of Bristlecone pine, a tree with a very long life span, have been dated using both dendrochronology and radiocarbon dating. The results do not agree, but the differences are consistent. That is, the radiocarbon dates were always wrong by the same number of years. Consequently, tree-ring chronologies have been used to calibrate radiocarbon dates to around 12,000 years ago.

When radiocarbon dating was first put into use, it was decided that dates would always be reported as B.P., where B.P. stood for “before present” and  “present” was defined as 1950. That way, dates reported in magazine articles and books do not have to be adjusted as the years pass. So if a lab determines that an object has a radiocarbon age of 1,050 years in 2000, its age will be given as 1000 B.P. Calibrated dates are given using the actual date,  such as 950 C.E.

Potassium-Argon Dating.

 If an object is too old to be dated by radiocarbon dating, or if it contains no organic material, other methods must be used. One of these is potassium-argon dating. All naturally occurring rocks contain potassium. Some of the potassium in rocks is the radioactive isotope potassium-40. Potassium-40 gradually decays to the stable isotope  argon-40, which is a gas. When the rock is melted, as in a volcano, any argon gas trapped in the rock escapes. When the rock cools, the argon will begin to build up. So this method can be used to measure the age of any volcanic rock, from 100,000 years up to around 5 billion years old.

 This method is not widely used in archaeology, since most archaeological deposits are not associated with volcanic activity. However, Louis and Mary Leakey successfully used the method to determine the ages of fossils in Olduvai Gorge in Tanzania by examining rocks from lava flows above and below the fossils. They were able to establish an absolute chronology for humans and human ancestors extending back two million years. At Laetolli, in Tanzania, volcanic ash containing early  hominid footprints was dated by this method at 3.5 million years.

Other Methods.

Uranium-238 is present in most rocks. This isotope of uranium spontaneously undergoes fission. The fission fragments have a lot of energy, and they plow through the rock, leaving a track that can be made visible by treating the rock. So by counting fission tracks, the age of the rock can be determined. Like potassium-argon dating, this can only be used to determine the age of the rock, not the age of the artifact itself.

Thermoluminescence is a recently developed technique that uses the property of some crystals to “store” light. Sometimes an electron will be knocked out of its position in a crystal and will “stick” somewhere else in the crystal. These displaced electrons will accumulate over time. If the sample is heated, the electrons will fall back to their normal positions, emitting a small flash of light. By measuring the light emitted, the time that has passed since the artifact was heated can be determined. This method should prove to be especially useful in determining the age of ceramics, rocks that have

been used to build fire rings, and samples of chert and flint that have been deliberately heated to make them easier to flake into a projectile point.

Conclusion

Science continues to develop new methods to determine the age of objects.

As our knowledge of past chronologies improves, archaeologists will be better able to understand how cultures change over time, and how different cultures interact with each other. As a result, this knowledge will enable us to achieve a progressively better understanding of our own culture.

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Reference

Aitken, M. J. Science-based Dating in Archaeology. London: Longman, 1990.

Baillie, M. G. L. A Slice through Time: Dendrochronology and Precision Dating. London U.K.: Batsford, 1995.

Brennan, Louis A. Beginner’s Guide to Archaeology.Harrisburg, PA: Stackpole Books, 1973.

Taylor, R. E. Radiocarbon Dating: An Archaeological Perspective. Orlando, FL: Acade-mic Press, 1987.

Taylor, R. E., A. Long, and R. Kra. Radiocarbon after Four Decades: An Interdiscipli-nary Perspective. New York: Springer-Verlag, 1994.

Wood, Michael. In Search of the Trojan War. New York: New American Library, 1985.